"Monster Manual" is a song by the band Mixel Pixel. It tells the tale of a role-player's struggle with a particularly brutal Dungeon Master, who is throwing just about every creature in the Dungeons & Dragons Monster Manual at him. The video is animated by Dan Meth, whose work has been featured extensively on Neatorama.

Chromatic dragons are single-headed (usually - I guess mutants or wizard experiments can exist). What I believe you are thinking of is the Goddess of chromatic dragons, who's form is a dragon with five chromatic heads (one for each chromatic dragon type - black, white, red, blue, and black). Often goes by the name Tiamat, but has shown up under other names in different settings (Takhisis, for example).